LCDs, which are used as optical display devices, are operated in an indirect manner so that an image is displayed by controlling the transmissivity of an external light source, and a backlight unit is used as an external light source and is an important part determining the characteristics of LCDs.
In particular, with the advance of LCD panel manufacturing technologies, high-brightness thin LCDs are increasingly required, and thus various attempts to increase the brightness of a backlight unit have been made. The barometer of excellence of LCDs, which are used for monitors, personal digital assistants (PDAs), notebooks and the like, is to exhibit high luminance using a small amount of energy. Therefore, the front brightness of LCDs is very important.
Since LCDs have a structure in which light having passed through a light diffusion layer is diffused in all directions, the amount of light radiating in the front direction of the LCD is very insufficient, and thus attempts to exhibit high brightness using low power consumption are being made continuously. Further, for large-area displays, attempts to enlarge a view angle allowing many users to easily view them have also been made.
However, when the power of a backlight unit is increased in order to enlarge the view angle, power consumption is increased, and thus power loss is also increased by heat. Therefore, in the case of portable displays, battery capacity is increased, and battery life is decreased.
Accordingly, in order to increase brightness, methods of providing orientation to light have been proposed, and, for this purpose, a variety of lens sheets have been developed. As a typical example of the lens sheets, there is a prism sheet having a prism structure, that is, a structure in which several ridges and recesses are linearly and regularly arrayed.
Here, the prism structure is a triangular array structure having an incline for improving brightness in a front direction. However, the prism sheet is problematic in that since the prism structure is provided with several ridges, the ridges are easily broken or abraded by weak external scratching, thus damaging the prism structure. Since the angles output from prism structure having the same shape are identical with respect to each array, brightness is reduced and defects are formed due to the differences in the light path between a damaged portion and a normal portion even when the corners of a triangle are somewhat crushed and the inclines of the triangle are slightly scratched. Therefore, the front face of the produced prism sheet may not be used depending on positions where even slight defects are formed at the time of producing the prism sheet. Thus, productivity is decreased, and production costs are increased. Really, even in enterprises assembling backlight modules, there is a problem in that, at the time of treating the prism sheet, the prism structure is damaged by scratching, thus causing defects.
Further, when a backlight unit is mounted with a prism sheet, the work of laminating several sheets and films is conducted. In order to increase brightness, a plurality of prism films may be mounted, and, in this case, there is a problem in that the upper portion of a lower prism film comes into contact with the lower portion of an upper prism film, thus easily damaging a prism structure.
Moreover, in consideration of brightness improvement, concealing ability and view angles, optical sheets having a structural form are practically used. When these optical sheets are mounted in a backlight unit, the work of laminating these optical sheets with other sheets and films is conducted, and, in this case, there is a problem in that the upper portion of a lower prism film comes into contact with the lower portion of an upper prism film, thus easily damaging a prism structure. Further, it is required to be cautious at the time of transporting or treating these optical sheets.
Therefore, in order to prevent such a structure from being damaged, conventionally, a protective film is additionally provided. However, as LCD panels become thin, the protective film is not used, and composite sheets are increasingly used. Further, since the process of laminating the protective film is additionally required, there are problems in that production costs are increased, and efficiency per time and physical efficiency are decreased.
In spite of the damage of the prism structure, portable displays, such as notebooks, personal digital assistants (PDAs) and the like, are increasingly used, and simultaneously most users move around with the portable displays in their bags and the like. In this case, when this portable display is shocked by the running during movement, sudden stops of vehicles or the like, a prism structure mounted in the portable display is damaged even though the portable display has a protective film, thus badly influencing a screen.
Meanwhile, in order to solve the above problems, an optical sheet was manufactured using a material having excellent elasticity. However, this optical sheet is also problematic in that sheets closely adhere to each other due to the sticky properties of a resin, thus forming defects or spots.